Dynamic DNS, or dynDNS, is a service that allows your computer to be reached by name whenever and wherever it's connected to the internet. (Every computer should have one :)

See http://directory.google.com/Top/Computers/Software/Internet/Servers/Address_Management/Dynamic_DNS_Services/ for a more current list of dyndns providers and related sites in a well ordered list, as this page is no longer actively maintained.

See http://www.technopagan.org/dynamic/ for more a more thorough introduction to DYNDNS, and more independant reviews of DYNDNS providers.

TTL stands for time-to-live, which is how long the IP address for the name is to be cached. Unless you have a static-ip, you probably want a TTL of a about a minute or less, otherwise longer.

There are several places that offer dynDNS:

Dyn Service Cost TTL in seconds Comments
DynIP.com free for 30 days, then $20/year. 60 Whenever they think you're offline, your .dynip.com address will point to their web server.
Dynamic DNS Network Services free 4 The signup is quick and automated. You can choose from .dyndns.org .mine.nu .homeip.net .ath.cx .itacs.to .merseine.nu .shacknet.nu
Domain Host Services free 600 Is finally offering DynDNS services!
Elmit.com $5/mo Great support for Chineese and German. Based in Taiwan.
Pronym.com free adjustable A, MX, CNAME records, and offline mail services available
ZeroPain.com free zeropain.com, trekaholic.com, suckMyEmail.com, pleaseDontMakeMeKillYou.com, and a few others
DynoDNS free 10
www.no-ip.com free 60
cjb.net 0
Dtdns.net free 15 Can be updated manually, or with provided auto-update clients. Also, you have a choice between hostname.dtdns.net and hostname.darktech.org. Their signup looks quick and simple.
yi.org free 120 Run by the former owner(s) of nx.org
HN.org Dynamic DNS services free 60 free dns for any domain, not just .hn.org
Dynu Systems free 180
Tzo.com free for 30 days, then $20/year. 30
myIP.org free 5 Looks simple, friendly, good.
Eye-P.net free By crontab.org
www.dynamic-dns.de 79dm/yr 60
Nikhilino Online Systems $20/year 300 Mention oth.net and save $5 off your first payment
DnsWiz.com $100 for 2 years They do not provide the name, only the nameservice, i.e. you dont get to use a subdomain-of.dnswiz.com
www.name-keeper.com free
www.dhis.org free
Microtech.co.gg/dns free
Sites not presently offering DynDNS service:

dyns.cx free 300 Not responding.
DynDns.com $25/year 60 Not responding.
Detour.net was $24/year 30 No longer accepting accounts. Not enough interest in the project...
EnterZone was $25/year Now closed to public.
NetWorld Internet Services was free Gone now.
DDNS.org was free 600 Slow site.. looks like they are down for now.
Monolith / ml.orgwas freeThe dearly missed biggest best first longest running original free public dyndns site ever.